1. Field of the Invention
This invention pertains to a procedure for switching-over systems for use in a duplexed ATM exchange operating in an asynchronous transfer mode (hereafter referred to as an ATM exchange).
2. Description of the Related Arts
Research and development has been actively conducted for realizing B-ISDN (Broadband aspects of Integrated Services Digital Network) for providing high-speed broadband communications such as full-fledged video image communications, transmissions of video signals of high-definition televisions and transfers of large files.
ATM (Asynchronous Transfer Mode) is a transmission/exchange mode forming a nucleus of the B-ISDN. Under ATM, a fixed length data column having fifty-three [53] bytes (octets) called an ATM cell is a unit of multiplexing and exchange. An ATM exchange network builds itself upon two [2] stages called a virtual channel (VC) and a virtual path (VP).
A VC, corresponding to a line, is a minimum unit of a logical connection the routing of which an ATM network processes. A VP, corresponding to a digital path, is a large bound of logical connections whose routing an ATM network processes, and is a multiplexed value of VCs. Therefore, a single VP connection must have at least one [1] VC connection.
An ATM exchange is an exchange for repeating and exchanging the ATM cells in an ATM network. When used as an LS (Local Switch: a subscriber line exchange) in an STM (synchronous transfer mode) network, it is connected to a subscriber line. An ATM exchange selects an individual VC connection by a VCI (Virtual Channel Identifier) stored in the header of such an ATM cell, and exchanges/repeats the ATM cell based on the value of its VCI. And a VP connection is identified by a VPI (Virtual Path Identifier) also stored in the header of such an ATM cell. In other words, an ATM network has a VPI and VCI to identify a particular cell between unique locations.
Incidentally, an ATM network specifically calls an ATM exchange forming a VP network an ATM cross-connect. The ATM cross-connect processes VC connections transparently. The values of VPI and VCI are determined when an ATM connection is established, and are retained until the ATM connection is released. The establishment and release of an ATM connection uses a signaling procedure, for example. A virtual channel connection different than one for user data called a signaling virtual channel carries the signaling data necessary then.
An ATM exchange has an internal switching unit called an ATM switch to process a routing. Hardware processes the routing control by referring to a tag attached to the head end of a header of an ATM cell (hereafter referred to simply as a cell).
Incidentally, the ATM exchange needs to duplex its switching units for improving its reliability. However, when an ATM network forms a duplex structure comprising an active (master) system and a backup (slave) system, because these two [2] systems are not synchronized, a cell transmission timing difference arises between these two [2] systems. Accordingly, when the systems are simply switched over during a maintenance or an operation, a cell may be duplicated or lost. That is, when the active system has a smaller transmission delay than the backup system, a simple system switchover causes a cell to be duplicated. Alternatively, when the active system has a larger transmission delay than the backup system, a simple system switchover causes a cell to be lost. Even though a synchronization of these two [2] systems will prevent cells from being duplicated or lost, this destroys the merit of an ATM network.